Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe that serves as a transmission line in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
One type of probe used by GWR level instruments is a coaxial probe. The coaxial probe consists of an outer tube and an inner conductor. When a coaxial probe is immersed in the liquid to be measured, there is a section of constant impedance, generally air, above the liquid surface. An impedance discontinuity is created at the level surface due to the change in dielectric constant of the liquid versus air at this point. When the GWR signal encounters any impedance discontinuity in the transmission line, part of the signal is reflected back toward the source in accordance with theory based on Maxwell's laws. The GWR instrument measures the time of flight of the electrical signal to, and back from, this reflecting point, being the liquid surface, to find the liquid level.
With any steam vessel, thermal energy is lost to the surroundings due to thermal conduction through the tank walls and convection around the exterior of the tank. If enough heat is lost, then the saturated vapor in the vessel will condense on the interior walls. Condensation can also occur in the GWR probe P located on top of a tank T as shown in FIG. 1. Particularly, there is a space S between the center conductor and the outer sleeve. Heat lost is represented by the arrows H. Because of the heat lost, the space S may be partially filled with condensation. Because the dielectric constant of water condensation is much greater than the vapor, the presence of condensation in the space S of the probe P may cause a significant delay of the signal transmission and result in a level measurement error.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.